To test connectivity from a whitelisted IP, you may simply:

Using your browser, visit the sandbox environment or the production environment . (Note: a whitelisted IP for the sandbox environment may not work in the production environment and vice versa.)
You will be prompted for a username and password by your browser's Basic authentication mechanism.
Enter your API Key for the username.
Enter your API Secret for the password.
If connectivity is successful, you will get a valid json response containing a welcome message.

The API key and Secret are available via the Helix Admin site or from a Helix representative.